Protection device for hammer driving circuits

ABSTRACT

The protection of a plurality of current pulse circuits, for example of the type used to activate high speed print hammers, is provided by a device which monitors the voltage levels at an appropriate terminal of each circuit. If the voltage level is not within predetermined limits in a suitable time interval, a signal is generated by the device which interrupts the power supply, thereby preventing damage to the circuitry, and/or activates an alarm. This device can also detect certain types of faults before the current pulse circuits have been energized.

United States Patent [1 1 Vigini Y 1 July 24, 1973 [54] PROTECTION DEVICE FOR HAMMER 3,489,920 1/1970 Moore et a1. 317/31 X DRIVING CIRCUITS 3,535,591 10/1970 l-lolmquest 317/31 X [75] Inventor: Giorgio Vigini, Milano, Italy Primary Examiner-James D. Trammell [73] Assignee: Honeywell Information Systems Inna, Caluso Italy. Attorney-Fred Jacob et al.

[22] Filed: July 12, 1971 [21] Appl. No.: 161,580 [57] ABSTRACT, I

The protection of a plurality of current pulse circuits, [30] Forelgn Apphcamn Pnomy Data for example of the type used to activate high speed July 30, 1970 Italy 28063 A/70 prim hammfirs, is provided by a device which monitors 1 the voltage levels at an appropriate terminal of each [52] 317/31, 317/27 317/33 circuit. If the voltage level is not within predetermined 317/36 TD limits in a suitable time interval, a signal is generated [51] hit. Cl. H0211 3/24 by the device which interrupts the power supply, [58] Field of Search 317/31, 27 R, 33 SC, thereby preventing damage to the circuitry and/or 317/36 TD tivates an alarm. This device can also detect certain types of faults before the current pulse circuits have [56] References Cited been energized UNITED STATES PATENTS 3,575,107 4/1971 McDowell 317/31 X 7 Claims, 4 Drawing Figures ELECTROMAGNETS 0.0. POWER SUPPLY CONTROL DEVICE VOLTAG E COMPARATOR PAIENIED 3. 748.537

sum 1 or 2 rlllullllllllllllllllllllllllnlll ll mfiz sompowfi lllllll |.l .I M l l l I I .I ll IL Giorgio VIC/NI INVENTOR ATTORNEY PATENKUJUWQ 3.748.537

SHEEI 2 BF 2 B A llllil Illlllllllll l i SC I F|G.2 STABILIZED POWER SUPPLY ELECTROMAGNETIC 10s 10s 0 109 R l5 1 SUPERVISOR CIRCUIT I 1 161 170 FIGJ GJorgio V/G/NI INVENTOR ATTORNEY BACKGROUND OF THE INVENTION The instant invention relates to a device for protecting circuits subject to high amplitude, short duration, current pulses, and more particularly to a device for the protection of the energizing circuits of the electromagnets which drive the printing hammers in high speed printers of electronic data processing systems.

The circuits for energizing the electromagnets, which drive the printing hammers, are protected at present by rated fuses, connected in series, in the control circuit of the electromagnet. The rating of the fuses must be such as to allow the fuses to function in the presence of high amplitude, short duration current pulses, but cause the fuses to become open-circuited in the presence of current pulses of longer duration but much lower amplitude.

However, a repeated succession of high-intensity, short-duration current pulses may cause an alteration of the physical and chemical structure of the fuse, so that the fuse may become open-circuited for current pulses of the allowed amplitude, thereby putting the printer out of service and signalling a non-existent defect.

On the other hand, the overrating of the fuses may eliminate their protection even in case of dangerously large steady currents.

It may be added that protecting the energizing circuit of each hammer-driving electromagnet of a parallel printer, which may comprise 160 or more hammers, involves high costs, low reliability and long repair times, time that may be employed in the search for a nonexistent defect.

The object of the present invention is therefore to improve the operation and the maintenance of high speed printers and other current pulse devices, by protecting the related circuits with a simple, reliable and nondestructive device.

SUMMARY OF THE INVENTION that the voltage of a suitably chosen point of the ener-' gizing circuit does not remain at a value lower than a predetermined limit for a time larger than a predetermined amount.

In any case, the monitoring of the permanence of such conditions on a plurality of energizing circuits is made by a single protection circuit. This protection circuit stops the operation and provides an alarm signal when a failure is detected in any one of the monitored circuits.

BRIEF DESCRIPTION OF THE DRAWINGS The invention .will be described with reference to the accompanying drawings, wherein:

FIG. I is the wiring diagram of the device of the present invention, together with a simplified wiring diagram of a circuit for the cyclic charging of the energizing circuits of the electromagnets, and with the logical block diagram of the control circuit of the charging circuit.

FIG. 2 is a diagram showing the waveforms of voltages and signal voltages during a printing cycle.

FIG. 3 is the simplified wiring diagram of the device in case of continuous charging of the electromagnet energizing circuits.

FIG. 4 is a diagram showing the waveforms at various points of the circuit of FIG. 3.

DESCRIPTION OF A PREFERRED EMBODIMENT The present description is particularly relevant to the case of a plurality of electromagnets for driving the hammers of a high-speed parallel printer. Each hammer is driven by the abrupt discharge of a storage capacitor. This discharge is controlled by a switching device, which may be for example a power transistor, or

a Silicon Controlled Rectifier (SCR). The latter is known to be a semiconductor device which may be considered as a silicon rectifier provided with a control electrode. Although biased in the forward direction, the rectifier, does not conduct in its quiescent state. However, a pulse of suitable polarity applied to the control electrode causes the current to flow from the anode to the cathode (SCR On). The rectifier reverts in the non-conducting state (SCR Off) when the flow of the current ceases or through superposition of an equal current in the opposite direction.

According to a first embodiment of the invention; the circuits, in which the discharge of the storage capacitor through the print electromagnet is controlled by Silicon Controlled Rectifiers, will be considered. The SCR goes off automatically when the current flowing through it ceases, in this instance, by the exhaustion of the charge of the storage capacitor. Accordingly, the storage capacitor must be charged during time intervals when the capacitor is not being discharged. The storage capacitor is recharged by means of a device operating periodically and called the cyclic charging circuit.

FIG. 1 shows:

the simplified wiring diagram of the circuit comprising the cyclic charging circuit and the electromag- I net energizing circuits;

the logical block diagram of the control device;

the wiring diagram of the protection device and its connection to the circuits to be protected, according to the invention.

In FIG. 1, a d-c power supply 1 delivers to the cyclic charging circuit a non-stabilized d-c voltage, for instance of 50 V. i 15%. The power supply is fed from an a-c network through a remote control switch 55.

The voltage delivered by the power supply I charges filter capacitor 2. The cyclic charging circuit indicated as a whole by reference numeral 3 comprises substantially: a two-winding inductor 4; a rectifier 11 in series with the second winding of this inductor; a charging SCR 6; a discharging SCR 8 connected in series to parallel combination of resistor 5 and capacitor 12; a cutoff capacitor 10 with a circuit for controlling the same, indicated as a whole by reference numeral and a stand-by capacitor 13 shunted by a resistor 14 and series connected to a diode 15. The output terminals 16 and 17 of the cyclic charger provides the energy for recharging storage capacitors such as 21, 31, 41

through the diodes such as 22, 32, 42 The charging circuit provides a precisely determined voltage at cyclically repeated time intervals during which printing does not take place. The energy of the storage capacitor is used for activating print electromagnets such as 23, 33, 43 under control of SCRs such as 24, 34, 44

The succession of operations is controlled by proper signals generated by a control device 51. The operation is synchronized with the period of rotation of the typecarrying device of the high speed printer not shown in the drawing. It is known that this rotation period, corresponding to a print cycle, is subdivided in two phases: a print phase, during which the printing hammers may be fired, and a line advance phase during which the paper is advanced by a number of line spacings. In this phase the recharging of the storage capacitors takes place.

The operation of the cyclic charging circuits will now be described briefly. At rest conditions SCR 6 and 8 are Off. Therefore there is no voltage across terminals 16 and 17, because no current flows through SCR 6 and capacitors l and 13 have been discharged as will be explained. At a suitable time of the line advance phase, the control device 51 sends a Charge Start signal (IC) (FIG. 2) to device 52 which is essentially a pulse generator and which in turn sends a firing pulse to SCR 6. SCR 6 becomes conductive, and permits a current flow to charge capacitors 13 and 10, as well as the storage capacitors 21, 31, 41 which have been discharged in energizing the corresponding electromagnets during the preceding print phase.

The first winding of the two-winding inductor 4 is therefore electrically connected to the capacitors, while the circuit of the second winding is interrupted because diode 11 is back-biased. Due to the series connection of the capacitances and the inductance in this circuit, the current flowing through the inductance has an oscillatory character. As a consequence, the voltage across terminals 16 and 17 goes from zero to a value higher than the voltage delivered by the power supply 1. The voltage across terminals 16 and 17 is measured by a standard voltage comparator device 54, where this voltage is compared to a predetermined threshold voltage, for example 60 V. When the voltage across terminals l6 and 17 reaches this value, the comparator 54 sends a Charge Stop (AC) signal to the device 53, a pulse generator capable of sending a firing pulse to the discharge diode, SCR 8. As the diode becomes conductive, the current delivered by the power supply may now flow through SCR 8 and resistor 5, which is in parallel with capacitor 12, instead of circulating through SCR 6.

The cut-off control circuit 80 consists of transistor 77, supplied by the voltage across terminals 16 and 17. The collector of transistor 77 is connected to resistor 78 which in turn is connected to terminal 16. The base of transistor 77 is connected to resistor 79 which is connected to terminal 16, to resistor 82 and capacitor 81 which are, in turn connected to terminal 17 as well as the emitter of transistor 77. The elements connected to the base of transistor 77 serve as a time dependent voltage divider. In rest condition the transistor 77 is off and remains in this condition during the phase of increasing voltage during which the capacitor 10 is charged. The current charging the capacitor 81 causes avoltage drop across resistor 79, thus maintaining the base of transistor 77 at low voltage level. As soon as SCR 8 becomes conductive, the capacitor 10 is discharged through resistor 5 and SCR 8. A current of opposite direction is superposed on the current formerly flowing through SCR 6 reducing the current to zero and turning SCR 6 Off. The transistor 77 remains Off for a short time, due to the time constant of the RC circuit comprising resistor 79 and capacitor 81. Then transistor 77 goes On, and capacitor 10 is rapidly discharged through resistor 78, causing the voltage across terminals 16 and 17 to decrease rapidly. Also capacitor 13 is discharged through resistor 14, and the diode 15 is back biased. The diodes such as 22, 32, 42 become backward biased, and capacitors such as 21, 31, 41 remain charged at voltage VC.

When the current in the circuit comprising the inductor-4, reaches its maximum value and starts decreasing, a forward biasing voltage for diode 11 is generated across the second winding of the inductor 4. The diode 11 becomes conductive, and a current flows in the sec ond winding, reacting on the first winding to prevent an excessive reverse voltage to be applied at the anodes of the SCRs. In addition, the current flowing through the second wind ing of inductor 4 contributes to the recharging of the filter capacitor 2, partially discharged during the preceding phase, and thus part of the ex pended energy is recovered.

At the end of the print phase, not all of the print capacitors 21, 31, 41 are discharged by energizing the associated electromagnets. It may happen that, at the end of the printing phase, very few or none of the capacitors are to be recharged. The stand-by capacitor 13 provides for a minimum capacity always present in the circuit, ensuring the oscillatory characteristics of the circuits, and thus providing the wanted excess voltage across terminals 16 and 17. When the charging is terminated, the capacitor 13 is rapidly discharged through resistor 14. The voltage across the terminals 16 and 17 is also applied to the device 56 which generates an End of Charge signal, as soon as this rapidly decreasing voltage falls under a predetermined value VF.

FIG. 2 shows the diagrams of different signals, the waveform of the charging voltage, and the correspondence of the different phases of a print cycle.

Line AB represents the duration of a whole print cycle (for instance 55 milliseconds in case of a printing speed of 1,100 lines per minute). The segment AC represents the phase reserved for the paper line advance, during which no printing operation may take place, and which therefore may be used for charging the capacitors. This interval may be 18 ms for example. The Start of Charge signal IC is delivered by the control unit 51 as a zero-going front of a binary level SC after a suitable delay from the time A of the start of the line advancing phase; This delay permits all elements which have operated during the preceding phase to be restored to a rest condition.

Element CC of FIG. 2 shows the waveform of the voltage across terminals 16 and 17.

When the increasing voltage reaches the threshold value VC, which is higher than the voltage delivered by the power supply 1, the charging current is interrupted and the voltage falls rapidly to zero. When the voltage across terminals 16 and 17, monitored by voltage testing device 56 has fallen below a predetermined value,

7 VF, the Test Charge signal FC is emitted by the voltage testing device 56.

The above description of the constitution and operation of the cyclic charger is necessarily incomplete, as it is limited to those elements which are necessary to the understanding of the operation of the protection circuit which is the object of the invention.

During the print phase (from C to B, FIG. 2), the control circuit delivers, through the print command leads, the firing pulses to those SCRs, which control the electromagnets. The electromagnets are energized in order to execute, at predetermined instants, the printing of the letters which the rotation of the typecarrying drum has brought in the print position at this same instant.

As each letter may be printed in more than one print position, it may happen that a plurality of print electromagnets are fired simultaneously. For an electromagnet to be fired, the control circuit applies a print command pulse CS to the control electrode of the SCR concerned. As the circuit of SCR 24 becomes conductive, the capacitor 21 is discharged through the winding of the electromagnet 24, activating the armature which throws the printing hammer toward the paper. After the capacitor is discharged, the inductance of the electromagnet maintains the flow of the current in the same direction until this current reaches the zero value and tries to reverse its direction. At this time the SCR goes Off and the passage of the current in the reverse direction is prevented. The capacitor 21 is therefore negatively charged, and point P acquires a negative potential. As the print phase, the signal Start of Charge IC permits the cyclic charging circuit to charge the capacitor 21, to a positive value, together with the other capacitors that have been discharged. Therefore point P and other points such as P P acquire or maintain a positive potential equal to the threshold voltage VC. When the Charge Test signal PC is emitted all capacitors and all points such as P P P in case of correct operation are at potential VC.

The protection device, according to the invention, comprises as many diodes, such as 26, 36, 46 as there are print circuits to be protected. The number of these circuits may be rather high. For instance, each device may protect 32 circuits or more. To protect all print circuits of a parallel printer; an adequate number of protection circuits will be provided. The protecting device comprises a supervisor circuit, indicated as a whole by reference numeral 70. The supervisor circuit contains transistors68 and 73 (of the NPN type for example) where the collectors are fed from voltage source +V through resistors 64 and 63 respectively and where the emitters are connected to the ground.

The base of transistor 73 is connected through resistor 66 and through ZENER diode 69 to input terminal I.

Input terminal I is also connected to resistor 62, which is in turn connected to voltage source +V. input terminal I is further connected to the grounded capacitor 71, and the resistor 61, which is in turn connected to thecathodes of the diodes 26, 36, 46 The collector of the transistor 73 is connected to the base of transistor 68 through diode '67 and the collector of the transistor 68, drives the output terminal U through resistor 74. A resistor 65 is connected between the base of transistor 73 and collector of transistor 68, and a resistor 76 is connected between the base of transistor 73 and the ground. Finally a resistor 75, having a relatively high resistance value, joins the input terminal I to the base of transistor 68.

The values of the resistors 64, 65 and 76 are so chosen, so that the transistor 73 is off when at least one of the diodes 26, 36, 46 has its anode connected to a voltage lower than the voltage drop across the Zener diode 69 (which for instance may be 3 V). As a consequence, transistor 68 is On and the output U is at 0 V, which corresponds to binary value ZERO. On the other hand, if all diodes 26, 36, 46 have the cathode at a voltage sufficiently higher than this value, the base of transistor 73 acquires a positive voltage, through the Zener diode 69 and resistor 66, such that transistor 73 is On and transistor 68 is Off. Therefore the output terminal U is at a positive value, for example 5 V., corresponding to binary value ONE. During the whole print cycle, the output terminal U may vary between the binary values ZERO and ONE, according to the voltages of points P P P In the case of correct operation, when the charging operation is terminated at the time of signal AC, all capacitors 21, 31, 41 should be charged at voltage VC and therefore all points P are at the appropriate voltage, that is, 60 V. The output terminal U will be at binary ONE.

The output terminal U is connected to the input lead, d, of a flip-flop 57, having its clock input lead, c, connected to the output lead of the voltage testing device 56, which generates the Test Charge signal FC.

This flip-flop is so arranged that it remains in rest condition, and its output u is at a binary ZERO if its input d is at binary ONE at the moment when the Test Charge signal is applied to input 0. 0n the other hand, the output u goes over to binary ONE if the input d is at binary ZERO when at input 0 the Test Charge signal is applied.

This binary ONE signal is applied to the control lead of the remote controlled switch 55, and causes the switch to open, cutting off the power supply to the whole device, and giving out an alarm signal by means of an auxiliary alarm device not shown.

Therefore the device detects, and signals as denoting a fault, the circumstance that one of the points such as P P P is at a potential approximately null, at the moment of the emission of the Test Charge Signal. At this moment, in case of correct operation, all these points should be at a positive potential. There are many reasons why point P may go to ground potential, for example the shorting of SCR 24, its failure to go Off in response to the cut-offsignal, the accidental grounding by a fault in insulation of a point of the winding of the electromagnet 23, the shorting of capacitor 27, the inability to charge the same because of defective operation of the cyclic charging circuit or by an interruption of the circuit branch containing diode 22, the shorting of diode 22,- etc.

The device is in addition capable to detect the fault consisting in the short-circuit of one of the diodes 26, 36, 46 In this instance, the 60 V. voltage is applied directly to input terminal 1, and through the resistor 75, a positive voltage is applied to the base of transistor 68, sufficiently high for saturating that transistor so that the output terminal U is at 0 V. During the normal operation, with normal diodes, the voltage applied to terminal l is derived from the voltage source +V and is of few volts. Therefore its influence on the base of transistor 68 through the high resistance resistor 75 is negligible.

The device may also detect a fault which exists before the cyclic charging circuit is switched on. In fact, when the voltage source +V is applied, this voltage, through resistor 61 and diodes 26, 36, 46 is applied to points P P I and charges the capacitors 21, 31, 41 even if the charging circuit is not operating. If the components of the print circuit are unimpaired, the points P assume a positive potential, along with terminal I, and therefore the output at terminal U is at binary ONE.

If one of points P is grounded, due to one of the above mentioned faults, the input I becomes V, and the output of terminal U is a binary ZERO. This value is applied to the input lead d of flip-flop 59. A clock input c, a validating signal SC is applied to flip-flop 59 by the control circuit a predetermined time, for example 30 seconds, after switching on the cyclic charging circuit. In case of a pre-existent fault a binary ZERO is applied to the input d, the output u of flip-flop 59, which is connected as an OR logic circuit to the output u of flip-flop 57, becomes a binary ONE and opens the switch 55, thereby switching Off the power supply. Thus the signalling of a pre-existent fault does not depend from the operation of the printer, and operation is prevented in the presence of certain fault conditions.

To remove an excessive sensitivity of the monitoring device, which could cause its output to change from ONE to ZERO and vice versa in response to small disturbances, a resistor 65 connects the collector of transistor 68 to the base of transistor 73. If the output U is a binary ONE and therefore the transistor 68 is Off, the voltage divider formed by resistors 64, 65 and 76 forward biases the base of transistor 73, stabilizing it in the on condition. If the output U is a binary ZERO, and therefore transistor 68 is On, the base of transistor 73 is grounded through resistor 65, and transistor 68, thus maintaining the base of transistor 73 at a low potential and making its passage to the On condition more difficult. By this negative feedback, two different threshold values are established, an upper one for the transistor 68 going over from Off to On condition, and a lower one for the opposite transistion. Greater stability and reliability of operation is thereby obtained.

The same monitoring circuit may be utilized without substantial modifications in the event that the charging device is not of the cyclic type, but has continuous operation. This circumstance is schematically represented in FIG. 3. A standard stabilized d-c power supply 101, is employed for maintaining across the output terminals 102 and 103 at a constant voltage VC for charging the print capacitors. This figure shows only one print circuit, comprised of diode 104, capacitor 105 and electromagnet 106. Other identical circuits are intended to be parallel connected to terminals 102 and 103.

In case of uninterrupted charging it is more convenicnt to employ a transistor as means for controlling the current passing through the electromagnet 106. The transistor is operated by a print signal ST sent out by the control device 151. The signal ST controls the switching of the transistors between the On and Off states at predetermined times. An SCR would require peculiar artifices to be cut off, if the printing circuits are being charged constantly.

According to the invention, point Q, corresponding to the collector of transistor 107, is connected to the input lead of an integrating circuit formed by resistor 109 and capacitor 110, connected in series between point Q and the ground. The point R, common to the capacitor and to the resistor, is connected to the diode 126 which has the same function as the diode 26 of FIG. 1. Other diodes 136, 146 are connected to corresponding points R of other integrating circuits which in turn are connected to points Q of other print circuits, which are not shown. The anodes of these diodes are connected, through resistors 161, to the input terminal I of the supervisor circuit 170, which is substantially identical to the monitoring circuit of FIG. 1.

The output U of this circuit, through an inverter 120, is applied to the remote control switch 155 which controls the on and off switching of the stabilized power supply 101.

FIG. 4 shows the waveforms of the voltages at points Q and R of. the print circuit of FIG. 3.

When transistor 107 goes On in response to a signal from the control device, the voltage of point Q decreases to substantially 0 V., and remains at this value as long as transistor 107 is On. When the transistor reverts to the Off condition, the voltage increases to a value somewhat higher than the initial value, and a short time after, it resumes the initial value. The time during which the transistor is On is the operating time of the electrogmagnet for practical purposes, and is always very short, of the order of a few milliseconds. By suitably choosing the values of resistor 109 and of capacitor the voltage of point R will not drop during this On interval below a predetermined threshold value VS, close to 0 V.

On the other hand this threshold value will be reached if there is a failure in the power supply circuit preventing the charging of capacitor 105, or if there is a fault in the print circuit of'the type listed above which maintains point Q sufficiently close to 0 V. The threshold voltage VS is chosen to be substantially coincident with the lower threshold input voltage of the supervisor unit 170, when a voltage less than this threshold is applied to input I, the output U of circuit 170 will become a binary ZERO. Therefore if no one of the points R connected to the input terminal I of circuit 170 falls below the threshold value VS, the output U of circuit 170 is a binary ONE, and the binary value applied to the control input of the switch is ZERO. If on the other hand the voltage of at least one of the points R, because of a failure, falls below the threshold value VS,

the output terminal U goes to binary ZERO, and a binary ONE is applied to the control input of switch 155,

causing the power supply switched off and giving out an alarm signal.

What is claimed is:

1. A device for protecting current pulse circuits coupled to a voltage source by a controllable switching means, comprising:

circuit means for checking a voltage level of said switching means, an alarm and cut off means for producing an alarm signal and cutting off said voltage source, whenever said voltage level of said switching means is not between an upper and a lower threshold value during predetermined time intervals.

2. The device of claim 1, whereby said current pulse circuits are charged at recurrent intervals of a temporal cycle by a cyclic device, said alarm signals being given out and said voltage source being cutoff whenever said voltage level of said switching means is not between said voltage level of said switching means remains lower than a predetermined value for a time interval longer than a predetermined value.

4. A device for protecting a plurality of firing circuits of a high speed printer, each firing circuit comprising a storage capacitor, a print electromagnet and an electronic switch connected in series with said print electromagnet, said storage capacitor being charged at recurrent time intervals by a cyclic charging circuit unit, the protecting device com-prising a supervisor circuit having an input terminal and an output terminal, the input terminal being connected to the anodes of an associated plurality of diodes, the cathode of each one of said diodes being connected to the connection between said print electromagnet and said electronic switch of an associated print circuit, said supervisor circuit delivering at its output terminal in succession a first and a second binary level, said first binary level being delivered when the voltage of at least one of said leads connected to said cathodes of said diodes is not between predetermined threshold values.

5. A device for protecting current pulse circuits, comprising circuit means for checking the voltage of suitable points of said circuits, and alarm and cut-off means for producing an alarm signal and cutting off said circuits, whenever the voltage of any one of said points is not between an upper and a lower threshold value during predetermined time intervals, whereby said alarm signal is given out and said circuits are cutoff whenever the voltage of any one said points remains lower than a predetermined value for a time interval longer than a predetermined value, and whereby a cyclic charging unit delivers a Test Charge signal after the end of the charging of storage capacitors, comprising a bistable, binary switching means having at least a first and a second input lead, and at least an output lead, for delivering on said output lead an alarm and cut-off signal in response to the coincident application of the Test Charge signal to said first input lead of a first binary level signal to said second input lead.

6. A device for protecting current pulse circuits, comprising circuit means for checking the voltage of suitable points of said circuits, and alarm and cut-off means for producing an alarm signal and cutting off said circuits, whenever the voltage of any one of said points is not between an upper and a lower threshold value during predetermined time intervals, whereby said current pulse circuits are charged at recurrent intervals of a temporal cycle by a cyclic device, said alarm signals being given out and said circuits being cut-off whenever the voltage of any one of said points is not between said threshold values during predetermined recurrent time intervals of said cycle, and whereby a validation signal is deivered at a short predetermined time after activating said cyclic charger, comprising a bistable switching means, having at least a first and a second input lead, and at least an output lead, for delivering on said output lead an alarm and cut-off signal in response to the coincident application of said validation signal to said second input lead and of said first binary level signal to said second input lead.

7. A device for protecting a plurality of printing firing circuits of a high speed printer, each firing circuit comprising a storage capacitor, a print electromagnet, and an electronic switch connected in series with said print electromagnet, said storage capacitors being steadily maintained at a predetermined voltage by a stabilized direct-current power supply, the device comprising a supervisor circuit having an input terminal and an output terminal, the input terminal being connected to the anodes of an associated plurality of diodes, the cathode of each said diode being connected to the output lead of an integrating circuit comprising a resistor and a capacitor series connected, the input lead of each integrated circuit being connected to the connection lead between said'print electromagnet and said electronic switch of an associated print circuit said supervisor circuit delivering at its output terminal an alarm and cutoff signal whenever the voltage of the output lead of at least one of said integrating circuit decreases under a predetermined value. 

1. A device for protecting current pulse circuits coupled to a voltage source by a controllable switching means, comprising: circuit means for checking a voltage level of said switching means, an alarm and cut off means for producing an alarm signal and cutting off said voltage source, whenever said voltage level of said switching means is not between an upper and a lower threshold value during predetermined time intervals.
 2. The device of claim 1, whereby said current pulse circuits are charged at recurrent intervals of a temporal cycle by a cyclic device, said alarm signals being given out and said voltage source being cut off whenever said voltage level of said switching means is not between said threshold values during predetermined recurrent time intervals of said cycle.
 3. The device of claim 1, whereby said alarm signal is given out and said voltage source is cut off whenever said voltage level of said switching means remains lower than a predetermined value for a time interval longer than a predetermined value.
 4. A device for protecting a plurality of firing circuits of a high speed printer, each firing circuit comprising a storage capacitor, a print electromagnet and an electronic switch connected in series with said print electromagnet, said storage capacitor being charged at recurrent time intervals by a cyclic charging circuit unit, the protecting device com-prising a supervisor circuit having an input terminal and an output terminal, the input terminal being connected to the anodes of an associated plurality of diodes, the cathode of each one of said diodes being connected to the connection between said print electromagnet and said electronic switch of an associated print circuit, said supervisor circuit delivering at its output terminal in succession a first and a second binary level, said first binary level being delivered when the voltage of at least one of said leads connected to said cathodes of said diodes is not between predetermined threshold values.
 5. A device for protecting current pulse circuits, comprising circuit means for checking the voltage of suitable points of said circuits, and alarm and cut-off means for producing an alarm signal and cutting off said circuits, whenever the voltage of any one of said points is not between an upper and a lower threshold value during predetermined time intervals, whereby said alarm signal is given out and said circuits are cut-off whenever the voltage of any one said points remains lower than a predetermined value for a time interval longer than a predetermined value, and whereby a cyclic charging unit delivers a Test Charge signal after the end of the charging of storage capacitors, comprising a bistable, binary switching means having at least a first and a second input lead, and at least an output lead, for delivering on said output lead an alarm and cut-off signal in response to the coincident application of the Test Charge signal to said first input lead of a first binary level signal to said second input lead.
 6. A device for protecting current pulse circuits, comprising circuit means for checking the voltage of suitable points of said circuits, and alarm and cut-off means for producing an alarm signal and cutting off said circuits, whenever the voltage of any one of said points is not between an upper and a lower threshold value during predetermined time intervals, whereby said current pulse circuits are charged at recurrent intervals of a temporal cycle by a cyclic device, said alarm signals being gIven out and said circuits being cut-off whenever the voltage of any one of said points is not between said threshold values during predetermined recurrent time intervals of said cycle, and whereby a validation signal is deivered at a short predetermined time after activating said cyclic charger, comprising a bistable switching means, having at least a first and a second input lead, and at least an output lead, for delivering on said output lead an alarm and cut-off signal in response to the coincident application of said validation signal to said second input lead and of said first binary level signal to said second input lead.
 7. A device for protecting a plurality of printing firing circuits of a high speed printer, each firing circuit comprising a storage capacitor, a print electromagnet, and an electronic switch connected in series with said print electromagnet, said storage capacitors being steadily maintained at a predetermined voltage by a stabilized direct-current power supply, the device comprising a supervisor circuit having an input terminal and an output terminal, the input terminal being connected to the anodes of an associated plurality of diodes, the cathode of each said diode being connected to the output lead of an integrating circuit comprising a resistor and a capacitor series connected, the input lead of each integrated circuit being connected to the connection lead between said print electromagnet and said electronic switch of an associated print circuit said supervisor circuit delivering at its output terminal an alarm and cut-off signal whenever the voltage of the output lead of at least one of said integrating circuit decreases under a predetermined value. 